The present invention is directed toward flexible sintered structures. More specifically, the invention relates to flexible high strength inorganic structures such as inorganic sheets or tapes, made by combining powdered metallic, metalloid or, most preferably, oxide powders with appropriate liquid vehicle components and casting or otherwise shaping and sintering the resultant powder batches. Sheets, foils, ribbons, or other high aspect ratio products made in accordance with the invention can exhibit high hardness, flexibility, and toughness with excellent thermal stability over a wide range of temperatures.
Thin flexible sintered structures are useful for a multitude of productive applications. They may be employed for electronic and/or electrooptic uses, such as waveguides, or as substrates for electronic coatings, superconductors, or high temperature superconductors.
With improved mechanical properties, flexible ceramics could be useful as a protective layer for glass or other substrate materials where a layer of protection is needed to resist scratches. With sufficient structural flexibility in the flexible ceramic, the object to be protected could simply be wrapped for protection.
Flexible inorganics, especially flexible ceramics, would offer unique advantages as chemically stable substrate materials. Porous ceramic materials are known to provide high surface areas. High surface area substrates provide desirable receiving surfaces for a variety of coatings. Alumina, for example, provides in its many crystalline forms an excellent surface for the application of catalysts. Porous or dense alumina which could be provided as a flexible ceramic foil and subsequently coated with a base or noble metal and/or oxide catalyst, or treated with zeolites, would have unique advantages for a variety of chemical applications.
Sintered porous metallic foils, e.g., porous stainless steel foils, can be made and optionally oxidized or otherwise treated to provide high surface area metal-based substrates. Coated substrates of metallic or oxide type, formed into any desired honeycomb or other circular, laminar, and/or trapezoidal structures, would offer stable support in harsh environments where flexibility in combination with a specific substrate geometry would be particularly advantageous.
Since the discovery of high temperature oxide superconductors, there has been widespread interest in combining these relatively brittle materials with strong flexible substrate materials to provide superconducting wires. Those skilled in the superconductor art have struggled to identify useful substrates for these superconductors.
One suggestion has been to use metallic components to provide supporting substrates or jacketing for the superconductors. A particular disadvantage of metals, however, is the diffusivity of the metals at the sintering temperatures required for ceramic superconductor application, which could undesirably modify the compositions of the applied superconductor materials.
Unlike metals, ceramic substrates are conventionally sintered at a higher temperatures than any of the yttrium barium copper oxide (YBCO), bismuth strontium copper oxide (BSCO) and/or thallium copper oxide families of high temperature superconductors, thus minimizing the diffusivity problem. Additionally, ceramics are more compatible with oxide superconductor coatings, due perhaps to improved wetting of the substrates by the coatings during coating application. Thus decreased interfacial discontinuities and increased substrate/layer stability are attainable. As those skilled in this art can appreciate, other metal and/or oxide and/or ceramic coatings would also benefit from this improved coating compatibility.
Of course the production of thin and flexible ceramic fibers such as silicon carbide fibers and aluminosilicate fibers is well known. Ceramic fibers of these types are generally produced by spinning techniques or variations thereof. For example, Nicalon.RTM. (silicon oxycarbide) fibers, Nextel.RTM. (Al.sub.2 O.sub.3 --SiO.sub.2 --B.sub.2 O.sub.3) fibers, and even .GAMMA.-alumina fibers are typically produced by spinning a fiber of a pyrolyzable precursor material and then pyrolyzing the spun fiber. Alternatively, fibers of alumina and zirconia can be produced by spinning a precursor material comprising fine oxide powder, followed by sintering to an integral oxide fiber product.
Still other methods of fiber manufacture include the vapor deposition of precursors onto a starting or substrate filament and/or the spinning and optional heat treatment of glass fibers from molten glass. Although none of the fibers produced from precursors as above described are perfectly cylindrical, almost all are of very low aspect ratio, i.e., below 2:1. For a further discussion of the major fibers and their use in composites, reference may be made to Frank K. Ko, "Preform Fiber Architecture for Ceramic-Matrix Composites," Am. Ceram. Soc. Bull., 68 [2] 401-414 (1989).
Unfortunately, while formed of inherently strong materials, long fibers of these ceramic materials are very weak. The weakness of fibers is simply due to the flaw populations in the fibers and the statistical laws which insure that most long fibers will include at least one defect of sufficient magnitude to cause failure at stress levels well below the inherent strength of the material.
While the strength levels attainable depend of course on the number and size of the defects introduced into the fibers from batch or manufacturing process sources, the defect population needed to sustain successful production of strong long fibers is very small. Thus, for example, it can be calculated that, for fibers of 10 microns diameter comprising defect particles or voids of similar size, defect levels below 1 defect per each one hundred million parts of volume are needed to yield reasonable selections of strong kilometer-long lengths of fiber.
Prior work in the field of thin film ceramics includes U.S. Pat. No. 4,710,227 disclosing the preparation of thin flexible "green" (unfired) ceramic tapes from solutions, the tapes being coated and cut, stacked and fired to form thin-dielectric capacitors. This process is further described in published European applications EP 0302972 and EP 0317676. Capacitors with ceramic layers of 1-50 microns can be made; however the capacitor fabrication process which is disclosed does not utilize the production or handling of sintered or fired (binder-free) flexible tapes in unstacked or unsupported form. In addition, the range of useful materials is limited by the ceramic process employed.